Since this came up in a google search, I'd like to post what worked for my 2002 Mercury Mountaineer, so no one has to pay the lame $7 following the link posted above.
I found this on a Google Forum:
For the '99's, the Owner's manual says -
1) Turn key from OFF to ON eight times within 10 seconds ending
in the ON position.
2) After doors lock then unlock, press any button on the
remote being programmed.
3) The doors should lock/unlock to confirm programming.
4) Turn key to OFF and door locks should cycle one last time.
You can program up to four remotes this way, but they must
all be done at the same time (repeat step 2 for each).
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http://groups.google.com/group/rec.a...&start=0&num=3
Hope that helps someone out there!